Based on test from https://code.google.com/p/chromium/issues/detail?id=497578
Currently fails. Basically, if there's a default_ignorable at the
start of text, and font has no space glyph, we remove the default_ignorable,
and that makes the first char in text to correspond to no cluster.
Fix coming.
Behdad Esfahbod
Ebrahim Byagowi